C# issues with generating sitemap.xml dynamically

Hello,

I want to generate sitemap.xml dynamically and having trouble reading the existing sitemap.xml and updating it. First step (code show bellow) is to clear the sitemap.xml file node urlset.

when code is executed, I'm getting the following error:
System.NullReferenceException: Object reference not set to an instance of an object.
selectedNode.RemoveAll();

One thing I noticed is if I clear urlset attributes it will work fine
<urlset
      xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9
            http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">
obviously, the problem is that it will not become a valid sitemap..

appreciate your help

string configPath = Server.MapPath("~/sitemap.xml");
        XmlDocument xmlDoc = new XmlDocument();
        xmlDoc.Load(configPath);
 
        XmlNamespaceManager nsMgr = new XmlNamespaceManager(xmlDoc.NameTable);
        XmlNode selectedNode = xmlDoc.SelectSingleNode("//urlset", nsMgr);
 
 
 
        selectedNode.RemoveAll();
        XmlElement nodeElmt = xmlDoc.CreateElement("urlset");
 
        xmlDoc.Save(Server.MapPath(configPath));

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